Conventionally, for investigation on an amount of marine resources, quantitating fish finders which detect a length of each single fish and display detection results on a display are known. Such kind of the quantitating fish finder is disclosed in JP2005-249398(A), for example. In this disclosure, the fish finder derives a ratio of a sound pressure level of an ultrasonic wave transmitted from a transducer and a sound pressure level of an echo which reflects on a fish (i.e., reflection intensity or target strength TS), and calculates a length of the fish (fish length L) based on the obtained target strength TS. Between the target strength TS and the fish length L, it is known that a relation of TS=20 log L+20logA is true (here, “A” is a coefficient determined by a signal frequency and a fish kind). The fish length L information on each single fish obtained by the quantitating fish finder is displayed in a sub display area provided adjacent to a main display area where so-called a fish finder image is displayed in a form of histogram for every predetermined fish size range; thereby enabling visual recognition of the measured values of the fish sizes.
JP2005-249398(A) displays by a predetermined graph the number of the measured fish (fish count) and the fish lengths which are measurement results, and by displaying a symbol of each single fish in the fish finder image of the main display area, visual recognition of a relation of the length of each single fish, a water depth, and an underwater distribution (refer to FIGS. 5(B) and 5(C) of JP2005-249398(A)) is substantially enabled. However, JP2005-249398(A) calculates the fish length and the fish quantity only from a viewpoint of quantitative observation, and carries out the graphical representation of the results in the sub display area, and information which lacks in the graphical representation are supplemented with the display contents of the fish finder image of the main display area. Therefore, in JP2005-249398(A), there is no anticipation and motivation of observing each swimming fish and no configuration therefor is described.